The present invention relates to a synchronous generator and a synchronous generator system driven by a windmill.
In order to use an AC-excited synchronous generator in a wind power generation system, it is essential to reduce the weight of the AC-excited synchronous generator as compared with the conventional counterparts. The requirement for weight reduction comes from the following reason. When a wind power generation system is constructed, the generator must be lifted up to the nacelle by a crane. In a wind power generation system using a large-sized windmill, the nacelle is located at more than 100 m above the ground level. Since the strength and therefore the diameter of the pillar for supporting the windmill is determined by the weight of the generator, the reduction of the weight of the generator is taken into much more consideration for the wind power generation system than for other types of power generation systems.
One way of reducing the weight of an AC-excited synchronous generator is to reduce the length of the air gap between the stator and the rotor. Reducing the air-gap length leads to reducing the magnetic resistance in the air gap so that the exciting current flowing through the field winding can be decreased. Accordingly, the cross section area of the conductor of the field winding can be diminished so that the resultant generator can be decreased in size and weight.
However, if the air-gap length is reduced, the spatial change in the magnetic resistance in the air gap becomes large due to the existence of slots in the stator and rotor so that the distortion of the armature current waveform is enhanced.
JP-A-3-270664 discloses a technique which is designed to improve the armature current waveform, wherein the numbers of the slots in the stator and rotor and the winding pitches for the stator and rotor are optimized to improve the waveform.
JP-A-7-15901 discloses a technique designed for improving the armature current waveform, wherein the winding pitches are so chosen that the winding factors relating to the slot-associated higher harmonics can be minimized.
JP-A-2005-304271 discloses a technique for suppressing the distortion of the armature current waveform by equating to ±6 the number of slots per two poles in stator minus the number of slots per two poles in rotor.